Beta ray detectors



Sept. 25, 1962 J. HERMSEN ETAL BETA RAY DETECTORS Filed July 28, 1959PULSE 24 INVENTORS JOHANNES HERMSEN KARS VAN D UUREN BY AGEN 3,056,059BETA RAY DETECTQRS Johannes Herrnsen, Amsterdam, and Kara van Duuren,Eindhoven, Netherlands, assignors to North American Phiiips (Iompany,Inc, New York, N.Y., a corporation of Delaware Filed July 28, 1959, Ser.No. Claims priority, application Netherlands Aug. 30, 1958 Claims. ((11.313-93) region near the window. Specifically, the field is attenuated atthe end of the wire-shaped anode opposite the window and is even weakerat the area of the transition of the window into the cathode. Hence theefiiciency of the beta ray counting is fairly low because much electronattachment occurs in the areas immediately behind the window, i.e.,between the window and the anode. The electrons that become attached tohalogen atoms and molecules are lost for the multiplication process.fore part of the incoming ,B-rays are not counted. Moreover electricalsurface charges may develop on the inside of an insulating windowproducing fluctuations in the counting efficiency and in the countersplateau characteristic, which is very undesirable particularly at lowcounting rates of a few counts per minute, which exist in determinationsconcerning radioactive fall-out.

In order to improve such counting tubes, it has been proposed to shapethe anode not in but in that of an elongated rod and to round-01f itsend opposite the window. Since such a rounding-off with an insulatingwindow would bring about a fairly large dead space, in which nodischarge can take place between 4.0 the form of a wire anode andwindow, it has been suggested also to make the window conductive, forexample by metallizing and to provide the same potential at it as at thecathode, for example, by connecting the metal coating directly to thecathode. While a counting tube thus constructed exhibits improvedeiiiciency for beta rays, it still has the disadvantage of a highbackground rate and a very small solid angle of collection, whichrenders such tubes essentially useless for counting low levelactivities, such as derive from fall-out.

The invention has for its main object to provide an improved countingtube for detecting beta rays in which the foregoing disadvantages areeliminated, while additional advantages are obtained, as will bedescribed hereinafter.

A Geiger-Muller counter according to the invention comprises a cathodewhich forms part of the envelope wall, an anode arranged inside thereof,and a substantially flat Window to allow beta rays to enter the activevolume. The tube is filled with a bet ray ionizable gas containing atleast 0.005% by volume of a halogen quench. The anode is a conductivebody, which is substantially spherical at least on the side facing thewindow and has in a plane parallel to the window a maximum dimensionbetween 0.15 and 0.75 times the largest dimension of the cathode in aplane parallel to the window. The window is conductive on the inner sideand electrically connected 3,055,059 Patented Sept. 25, 1962 has to thecathode. The largest height of the discharge space, measured at rightangles to the window, is between 0.25

the gas-filled window. The

background is materially reduced.

The known counter described in the introduction, which comprised acylindrical cathode, a rod-shaped anode and intense field is necessaryto prevent the occurrence of electron attachment, because of the strongelectro-negative character of the halogen atoms. These results, whichare not obtained in the known counter, are achieved in the inventivedevice, and a particularly favorable field distribution is establishedwhen the ratio between the largest dimension of the anode in a planeparallel to the plane parallel to the Window lies between 0.25 and 0.50.Further, the spherical shape of the anode should not be understood inonly a strictly mathematical sense. The anode may, for example, be anellipsoid of revolution or part thereof or, as an alternative, acylinder with a strongly rounded-off rim.

Since the anode is not wire-shaped, its external connection involvesdifliculties. If, for example, the anode is a hemisphere with a shortcylindrical portion secured thereto, this short cylinder must be sealedthrough the tube Wall so as to be insulated therefrom. Although, from atechnical point of view, the sealing of a fairly thick cylinder throughan insulator of, for example, glass is quite feasible, it is materiallysimpler to seal through a thin wire. In the case of a thick lead-in thestray capacitance of the counter is, moreover, increased, which affectsadversely its satisfactory operation as a counting device. Moreover, inthe case of a thick anode lead-in, the volume of the insulator is large.It has been found that this is also disadvantageous because a largerinsulator involves a greater number of spurious discharges, i.e.uncontrollable discharges occurring at any instant, than a smallerinsulator. This may be due to the fact that the insulator is charged byelectrons or ions from the gas discharge.

The disadvantages of a thick anode lead-in may be mitigated by securing,in a particular embodiment of a counter according to the invention, theanode on the side remote from the window to a supporting wire, which isat the same time a current supply wire, of which the diameter, at leastinside the discharge space, is smaller than one tenth of the largestdimension of a section of the cathode parallel to the window.

The supporting wire of the anode requires a greater height of thegas-filled space than without this wire, but with a choice of thethickness of the supporting wire as indicated above, it is possible toavoid the possibility that the gas-filled space portion between thesupporting wire and the cathode becomes sensitive to gamma radiation andcosmic radiation, since the starting voltage in the said portion, owingto the choice of the wire diameter, will be materially higher than inthe remaining portion of the discharge space. The supply voltage for thecounter can then be chosen such that the portion behind the anode bodyis inactive, i.e., does substantially not count. The thin wire can besealed through a small insulator, which will give rise to spuriousdischarges to a smaller extent.

The window may be made of different insulating materials, for examplemica or glass; on the inner side such a window may be made conductive byapplying, for example by evaporation, a thin metal layer, for example,of chromium. As an alternative, particularly with a glass window,conductivity may be obtained by means of a coating of conductive tinoxide. Instead of using an insulating window which is renderedconductive, the window may be made of a thin metal foil, for example, ofchrome-iron.

Although, as a rule, use will be made of a cathode which is cylindricalthroughout its length, it is advantageous, in order to obtain ahomogeneous field distribution, to round-ofl slightly the cathode endremote from the window.

The invention will now be described more fully with reference to thedrawing which shows several different embodiments of a counter accordingto the invention, which are drawn to scale with the dimensions derivablefrom the scale to the right of FIG. 1, with the exception of thethicknesses of the windows, which are shown on an enlarged scale forclaritys sake. In the drawing:

FIG. 1 is a cross-section of one embodiment of a counter according tothe invention, in which the anode consists of a solid hemisphere with asolid, short cylinder connected therewith;

FIG. 2 is a cross-sectional view of one embodiment of a counteraccording to the invention comprising a solid, hemispherical anode;

FIG. 3 is a cross-sectional view of one embodiment of a counteraccording to the invention comprising a nonsolid anode hemisphere,supported by a thin wire;

FIG. 4 is a cross-sectional view of one embodiment of a counteraccording to the invention comprising a solid spherical anode, supportedby a thin wire;

FIG. 5 is a sectional view of one embodiment of a counter according tothe invention comprising a solid anode shaped in the form of anellipsoid, supported by a thin wire.

As shown in FIG. 1, the cathode of the counter is formed by acylindrical wall 1, which may be made of chrome-iron (75% iron-25%chromium by weight). On one side this wall is closed off by a micawindow 2, which is coated on its inner side with a conductive layer 3,for example, of chromium, connected to the cathode 1. An anode 4 isconstituted by a solid body, for example, of chrome-iron; as shown inthis figure, this body 4 consists of a hemispherical portion at thebottom and a cylindrical portion connected therewith at the top. Thecylindrical portion is secured in the wall 1 by means of an annularinsulator 5, for example, of glass. The window 2, wall 1, insulator 5and anode 4 constitute a sealed envelope, whose interior is filled witha beta ray ionizable medium, such as a rare gas, and a small amount of ahalogen quench.

The aforesaid conditions relating to the dimensions of the variouscomponent parts of the counter according to the invention are clarifiedby designating by a the diameter of the window 2, by b the minimumdistance of the anode 4 from the window 2, by c the largest dimension ofthe anode in a plane parallel to the window, by d the largest dimensionof the cathode in a plane parallel to the window (in the embodimentshown in FIG. 1 d is equal to a, which, however, is not alwaysnecessary) and by f the largest height of the discharge space. Theaforesaid conditions can thus be represented as follows:

A specific example of a suitable geometry, which is not to be consideredas limiting, follows below:

the gas filling is a mixture of neon and argon with 0.05 volume percentof bromine at a combined pressure of mms. of mercury. The operatingvoltage is about 450 volts.

In FIG. 1 a battery is referred to by reference numeral 21 whichprovides the voltage for operating the tube. With reference numerals 22and 23 resistors are indicated placed in the supply circuit. The outputpulses are taken from the capacitor 24.

FIG. 2 shows another embodiment of a counter according to the invention,which is substantially identical to the embodiment shown in FIG. 1. Theanode 6, however, is a hemisphere and sealed directly to the insulator 7in turn sealed to the cathode 1.

As shown in FIG. 3, the cathode 8 may be rounded off spherically on theside remote from the window by means of a portion 9. In this portion 9the insulator 10, for example, of glass, is secured in a gastightmanner. The anode is a hollow hemisphere 11, which is secured in theinsulator 10 in a gastight manner by means of a thin wire 12. Thewindow, as in the embodiments shown in FIGS. 1 and 2, consists of ametal-plated mica foil 13, of which the thickness is about 0.01 mm.

In the embodiment shown in FIG. 4, the anode consists of a solid sphere14, which is secured in a gastight manner with the aid of a thin wire 15in the insulator 16, for example, of glass. The window of thisembodiment consists of a chrome-iron foil 17, the thickness of which isabout 0.02 mm. The diameter of the wire portion 15 inside the dischargespace, in accordance with the aforesaid conditions, in smaller than 0.1d(FIG. 1).

FIG. 5 shows a counter in which the anode 18 is an ellipsoid ofrevolution. This shape furnishes a still better field homogeneity at thearea of connection of the window 19 and the cathode 20.

Beta ray detectors in accordance with the invention have provedextremely suitable for low-level work, because of their extremely lowbackground and their high counting efiiciency. Moreover, the plateau areexcellent, being about 300 volts or larger and having a slope of 2% per100 volts or less. This illustrates the excellent characteristics of thetube (as is known, the tube is generally operated at a voltage at aboutthe center of the plateau, which is the horizontal portion of thecounting rate-voltage discharge characteristic). Moreover, short deadtimes are experienced.

What is claimed is:

1. A Geiger-Muller counting tube for the detection of cylindricalconductive surface serving as a cathode and a substantiallly flat, thin,beta radiatiomtransparent window portion having a conductive innersurface connected to the cathode and closing-elf the cathode and filledwith an ionizable medium containing at least 0.005% by volume of ahalogen quench,

dimension of the anode in a plane parallel to the Window being between0.25 and 0.50 times the longest dimension of the cathode in a planeparallel to the window, the height window, and means for applying apotential to the cathat Which the tube operates in the Geigerplateauregion of its counting rate-voltage discharge characteristic, wherebythe counting tube exhibits high efficiency together with low background.

6. A counting tube as set forth in claim 5 wherein the anode issupported on its side remote from the window by a wire whose diameterwithin the discharge space is less than 0.1 times the said longestdimension of the cathode.

7. A counting tube as set forth in claim 5 wherein the anode is asphere.

8. A counting tube as set forth in claim 5 wherein the anode is ahemisphere whose convex side faces the window.

9. A counting tube as set forth in claim 5 wherein the anode is anellipsoid of revolution whose major axis is parallel to the window.

10. A counting tube as set forth in claim 5 wherein the end of thecathode remote from the Window is curved.

References Qited in the file of this patent UNITED STATES PATENTS2,654,041 McCurdy et' al Sept. 29, 1953 2,837,677 Hendee et al. June 3,1958 2,917,647 Fowler et al. Dec. 15, 1959 2,985,785 sonk May 23, 1961FOREIGN PATENTS 740,500 France Nov. 14, 1932 472,110 Germany Feb. 22,1929 OTHER REFERENCES Theory and Operation of Geiger-Muller CountersII,by S. C. Brown, from Nucleonics, August 1948, pp. 64.

Low-Background Counter for Solid B-emitting Samples, by Softky et al.,from Nucleonics, May 1957, pp. 90, 92, and 93.

